Object motion during the acquisition of MR data produces both blurring and ghosts in the final image. Ghosts appear as equally spaced replicas of the moving object in the phase encoding direction. Collectively these replicas comprise a ghost mask. Ghosts are particularly apparent when the motion is periodic, or nearly so. For most physiological motion, each view of the MR signal is acquired in a period short enough that the object may be considered stationary during the acquisition window. In such case the ghosting is due to the inconsistent appearance of the object from view to view. The primary source of such ghosts is patient motion due to respiration or the cardiac cycle.
Ghosting has been considered problematic due to the potential for diagnostic misinterpretation when the ghosts are superimposed on the anatomical image. Previously, the attempt has been made to eliminate the ghost by methods such as gated MR scanning. Recently, a method for cancellation of image artifacts has been developed as described in U.S. patent application Ser. No. 07/718,678 to Xiang et al, filed Jun. 21, 1991 and incorporated herein by reference. The method employs a decomposition procedure to separate the ghost mask from the desired, unghosted image. The ghost free image obtained is the averaged spin-density distribution during the entire course of the scan. The method of U.S. patent application Ser. No. 07/718,678, requires the acquisition of three MR data sets. Further, a method has been discovered for producing dynamic images from motion ghosts. U.S. Ser. No. 08/015,103 of Xiang et al, filed Feb. 9, 1993 teaches a method for producing dynamic images based on the "three data acquisition" method of U.S. Ser. No. 07/718,678.
A method has now been developed for separation of motion ghosts by acquisition of only two MR data sets and processing. In addition, a dynamic image reconstruction method has now been developed based on the two data acquisition ghost suppression method.
The motion ghost suppression method and the dynamic imaging method disclosed herein avoids the need for monitoring motion and offers a higher data acquisition efficiency than conventional cine magnetic resonance imaging. Improved scan times and noise control are also provided.